Method of preparing photo-electric tubes



March 14, 1933.

K. T. BAiNBRIDGE METHOD OF PREPARING PHOTO ELECTRIC TUBES Original Filed Jan. 4, 1928 m r u b wmwm a w v. .t WT M mums e n. H w

Patented Mar. 14, 1933 UNITED STATES PATIENT OFFICE KENNETH ELECTRIC COMPANY, A. CORPORATION OF NEW YORK METHOD or PREPARING rnoro-nnnc'rmc TUBES Original application filed January 4, 1928, Serial No. 244,533. Divided and this application filed July 8,

. 1932. Serial No. 621,499.

The present invention relates to lightsensitive tubes and more particularly to tubes having a monatomic layer'of highly reactive metal and to a method for preparing a lightsensitive surface for such tubes.

This application is a division of my prior V application Serial No. 244,533 filed January 4, 1928, entitled Photo-electric tube, and assigned to the same assignee as the present application.

While light-sensitive tubes having a monatomic layer of light-sensitive material have hitherto been made, they have never gone into practical use due partly to the fact that their sensitivity has not been constant and partly to the fact that when made with highly reactive material, such as caesium or rubidium, the presence of these alkalies imairs the electric insulation of the tube. I have found that both of these difiiculties may be overcome by removing substantially all excess alkali metal from the light-sensitive tube and that photo-electric tubes made according to the present invention are not only stable and substantially free from electric leakage but are less sensitive to temperature changes than photo-electric tubes heretofore constructed.

In carrying out my coating is provided on invention, a silver the inner walls of an evacuated receptacle and this coating oxidized or provided with an adsorbed gas layer. A coating of caesium or other suitable alkali metal is then applied to the oxidized coat and all excess alkali metal removed byheating the receptacle to.an elevated temperature.

By excess alkali I mean all alkali metal that will evaporate at the evaporation temperature of bulk material.

Although I have found it desirable to pp y face such the benefits of the invention may be obtained by applyingalkali metal directly to the foundation metal. 'It is essential, however, to always remove substantially all excess alkali metal from the receptacle.

' The novel features which I believe to be characteristic of my invention are set forth the appended claims.

the alkali coating to an oxidized sura surface is not essentialas some of;

The invention itself, however, will best be understood from reference to the following specification when considered in connection with the accompanying drawing in which the single figure shows a perspective view of a photo-electric tube which embodies the features of the present invention.

Referring to the drawing, I have indicated at 1 a receptacle. provided with a pairof leading-in wires 2 and 3 between which a. small filament 4 is mounted. A silver bead, not shown, is initially supported and surrounded by the filament and is adapted to be vaporized when current is supplied to wires 2 and 3. A circular metal disc 6, mounted on leading-in wire 2, constitutes the anode member of the photo-electric tube. When current is supplied to the leading-in wires,- the silver bead is vaporized and a silver coating deposited on the bulb portion of the receptacle. Disc 6 prevents deposition of the silver on that portion of the bulb in the rear of the disc providing in this manner a window 5 for the tube. An insulating washer 7 of mica or quartz is mounted on wires 2 and 3 and prevents deposition of silver in the neck portion of the container. A wire 8 extends through the bulb portion of receptacle 1 and is connected with the metallic coating inner walls of the bulb portion of the receptacle, is oxidized. This is accomplished by admitting oxygen into the receptacle until a pressure of about 100 micronsof mercury is obtained. and thenglowing the bulb by applying a high frequency current thereto from a spark coil. The application of the spark coil to the bulb effects a clean-up of the free oxygen present in the receptacle. The use of, the high frequency coil is somewhat objectionable, however, owing to the fact that it may cause portions of the silver coating to be sputtered onto the window portion -5 of the receptacle. This difficulty may be overcome by admitting oxygen as above and then heating the bulb inan exhaust oven to a temperature of about 360 C. for a few minutes in- A plication of William or more capillaries silver.

'process but if removable member,

, greatly increased area of stead of applying the high frequency current to the receptacle.

When the desired degree of oxidation has been effected any excess oxygen is pumped out and a highly reactive metal, such as caesium, rubidium, potassium, or calcium, is distilled into the bulb portion of the receptacle. The reactive material, as for example cmsium, is preferably provided in sealed glass capillary tubes 9 which aremounted on a metal disc 11 and secured thereto by metal straps 12, as disclosed in the copending ap- E. Ruggles, which has matured into Patent No. 1,768,421 granted June 24', 1930. When the disc 11 is heated by a suitable high frequency current, the end portions of the glass capillaries crack and permit the caesium to flow out and into the bulb portion of the receptacle. The heating and opening of the capillaries is easily effected without shattering them. If desired, one

may be employed to obtain the proper amount of reactive material.

After the application of light-sensitive metal, such as caesium, to the oxidized metal surface, the receptacle is baked out at a temperature of about 300 C; for a period of time which may vary from a few minutes to about two hours depending on the amount of alkali metal present in the receptacle, the latter being in the meantime connected to a suitable evacuating pump. In this manner all excess caesium or light-sensitive material that did not stick monatomic layer of light-sensitive material covering all the free surface of the oxidized The amount of alkali thus adsorbed may be much greater than would be required to cover a smooth metal surface owing to the the spongy oxidized surface. If desired, lower temperatures than 300 C. may be employed in the baking out such temperatures are employed a longer time will be required to obtain a suitable layer ofv reactive material. Temperatures higher than 300 C. are usually undesirable as such temperatures might destroy the light-sensitive surface. The optimum temperature for removing excess light-sensitive material is different for different materials and Will be higheror lower than 300 C. depending upon the melting point of the element employed. For example, in the case of barium a temperatureof about 700 C. would be required to produce a monatomic layer. In this'case the receptacle maybe made of quartz. If desired, the light-sensitive surface mounted in an evacuated chamber and later mounted in a light-sensitive tube.

Photo-electric tubes having a light-sensitive surface prepared as indicated above, have a sensitivity about twice as great as that of the best potassium hydride, high vacis driven outleaving a may be prepared on a 'cess -caesium from the uum, photo-electric tubes heretofore constructed. For example, a tube constructed according to the present method gave a photo-electric current of 1.4 milliamperes as compared with currents of .680 and .650 milliamperes obtained from two potassium hydride tubes under similar operating conditions. Photo-electric tubes produced in the above manner have a high efficiency and are stable due to the absence of free alkali in the tubes. They are furthermore less sensitive to temperature changes than ordinary photoelectric tubes.

If desired, copper, nickel, tungsten, barium or other metals may be employed instead of silver as the base or foundation metallic layer. These metals may be deposited as thin films on the glass wall or mounted on stems in the form of platesor cylinders. Various alkali or alkaline earth metals may be em' ployed as the light-sensitive material and while I have employed the term alkali metal in the specification and claims, I intend that this term shall apply equally well to metals of the alkaline earth group,

It is my belief that the light-sensitive layer consists of alkali metal alone and not a compound, that is, when caesium is employed the tube comprises foundation metal, a monatomic layer of caesium and an intermediate layer of oxygen. The caesium surface is similar to the surface obtained on tungsten filaments which have been oxidized and then coated with caesium. Although different combinations of foundation metals and reactive metals may be employed, I have found that the best results are obtained when caesium is applied to a silver oxide surface.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. The method of preparing a light-sensitive coating on a metallic surface, which comprises forming an adsorbed gas layer on said surface, applying a light-sensitive metal layer to said gas layer and then heating said metallic surface and layers in a receptacle to a temperature sufliciently high to substantially remove all excess light-sensitive metal from said receptacle while said receptacle is being evacuated.

2. The method of preparing a light-sensitive coating to a metallic surface, which comprises applying a li ht-sensitive metal to said surface and then heating the surface in a container and at a temperature suficiently high to remove substantially all excess lightsensitive metal from the container while said container is beinv evacuated.

3. The metho of preparing a light-sensitive coating on a metallic surface, which coma layer of silver or other prises depositing caesium on said metallic surface, then baking the metallic surface in a receptacle and then pumping out all exreceptacle.

4. The method of preparing a li ht-sensitive coating .on a metallic surface, w ich comprises oxidizing the metallic surface, applying a reactive metal to the oxidized surface and then heating said surface in a receptacle to a temperature sufiiciently high to remove substantially all excess reactive metal while said receptacle is being evacuated.

- 5. The method of preparing a light-sensitive coating in a container, which comprises depositing a silver coating in the container, oxidizing said silver coating, distilling caesium onto the oxidized coating and baking the container while being evacuated to thereby drive out excess caesium.

6. The method of preparing \a light-sensitive coating in a container, which comprises depositing a foundation metal coating in the container, oxidizing said coating, applying caesium to the oxidized coating and heating the receptacle to a temperature of about 300 C. while being evacuated to thereby drive at excess caesium. v

7. The In thod of preparin a light-sensitive coating in a container w ich comprises depositing .a foundation metal coating in the container, applying a coating containing oxygen to said foundation coating, and applying caesium to said oxygen coating, and heating the receptacle to a temperature of about 300 C. while being evacuated to thereby drive out excess caesium.

In witness whereof, I have hereunto set my hand.

' KENNETH T. BAINBRIDGE. 

